There is nothing more annoying than getting in to a shower to find that the pressure is so low that the shower seems to only trickle the water out. Some people say that they would rather have a cold shower than one with no pressure. Some showers push out water in bursts so it constantly feels like you are having buckets poured over you rather than a nice decent shower. This can be for a number of reason, but often a lot of it has to do with the type of shower fitted and the location of the shower. If the shower is located above the water system then the water has to go against gravity to reach the shower head. The powerfulness of the shower also plays a big part as if it is not powerful enough it simply cannot move the water quick enough. If you have a combination of both these issues you may find your shower almost useless. It may also be linked to the shower head itself, if some of the jets are blocked up or if the shower is twisted round to make a jet rather than a spray then this will affect the way the water comes out.
All good bathroom and shower retailers should be able to advise you of what you need to improve the pressure and some will even come out to you to have a look at your set up. It may mean having to move the shower location or installing a different type.